Novel Oxidovanadium Complexes with Redox-Active R-Mian and R-Bian Ligands: Synthesis, Structure, Redox and Catalytic Properties.

A new monoiminoacenaphthenone 3,5-(CF)CH-mian (complex ) was synthesized and further exploited, along with the already known monoiminoacenaphthenone dpp-mian, to obtain oxidovanadium(IV) complexes [VOCl(dpp-mian)(CHCN)] () and [VOCl(3,5-(CF)CH-bian)(HO)][VOCl(3,5-(CF)CH-bian)]·2.85DME () from [VOCl(CHCN)(HO)] () or [VCl(THF)]. The structure of all compounds was determined using X-ray structural analysis. The vanadium atom in these structures has an octahedral coordination environment. Complex has an unexpected structure. Firstly, it contains 3,5-(CF)CH-bian instead of 3,5-(CF)CH-mian. Secondly, it has a binuclear structure, in contrast to , in which two oxovanadium parts are linked to each other through V=O···V interaction. This interaction is non-covalent in origin, according to DFT calculations. In structures and , non-covalent π-π staking interactions between acenaphthene moieties of the neighboring molecules (distances are 3.36-3.40 Å) with an estimated energy of 3 kcal/mol were also found. The redox properties of the obtained compounds were studied using cyclic voltammetry in solution. In all cases, the reduction processes initiated by the redox-active nature of the mian or bian ligand were identified. The paramagnetic nature of complexes and has been proven by EPR spectroscopy. Complexes and exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. The yields of products of cyclohexane oxidation were 43% (complex ) and 27% (complex ). Based on the data regarding the study of regio- and bond-selectivity, it was concluded that hydroxyl radicals play the most crucial role in the reaction. The initial products in the reactions with alkanes are alkyl hydroperoxides, which are easily reduced to their corresponding alcohols by the action of triphenylphosphine (PPh). According to the DFT calculations, the difference in the catalytic activity of and is most likely associated with a different mechanism for the generation of OH radicals. For complex with electron-withdrawing CF substituents at the diimine ligand, an alternative mechanism, different from Fenton's and involving a redox-active ligand, is assumed.